1. Field of the Invention
The present invention relates to an ink jet recording head substrate, and an ink jet recording head including the substrate.
2. Description of the Related Art
Typically, electrothermal conversion elements (heaters) of a recording head installed in an ink jet recording apparatus and a driving circuit and wiring for the heaters are formed on a same substrate using a semiconductor process technology, as disclosed in U.S. Pat. No. 7,216,960 (corresponding to Japanese Patent Application Laid-Open No. 2005-138428).
FIGS. 8 and 9 are schematic views illustrating an ink jet recording head substrate according to prior art.
FIGS. 8 and 9 illustrate the same substrate. FIG. 9 mainly illustrates heater driving power source wiring and ground wiring, and FIG. 8 mainly illustrates heater drivers and logic wiring underneath the heater driving power source wiring and ground wiring.
FIG. 8 is described first. An ink supply port 106 is provided at a center of the substrate, and a heater array 107 is arranged on both sides of the ink supply port 106. A driver array 501 for driving heaters is disposed behind the heater array 107, and logic wiring and logic circuit 502 are disposed behind the driver array 501. A plurality of connection pads 505 is arranged beside an edge of a shorter side of the substrate. Components such as a shift register 503, a decoder 504, and a temperature sensor (not illustrated) are provided between the ink supply port 106 and the plurality of connection pads 505.
In FIG. 9, heater driving power source wiring 603 is provided on the heater driver array, and ground wiring 604 is provided on the logic unit. The heater driving power source wiring 603 and the ground wiring 604 are connected to the outside via a heater driving connection pad 601 and a ground connection pad 602, respectively.
The power source wiring is divided in units of heater driving blocks, in order to ensure an approximately same wiring resistance for each arranged heater. Each wire has a different width depending on a distance from the connection pad so as to have a uniform resistance. Since the number of heaters simultaneously driven is 1 in any driving block, a voltage drop due to a wiring resistance is uniform in any heater.
In the case where the power source connection pads are provided only on one side of the substrate, the wire width increases excessively when the wiring is performed up to an opposite side of the substrate in the above-mentioned manner. Accordingly, the power source wiring on the substrate shows a symmetry between the upper and lower halves, as illustrated in FIG. 9. This requires a heater driving power source terminal and a ground terminal to be provided beside an edge of each of the two shorter sides of the substrate.
Pads other than the connection pads 601 and 602 of a power source system are used for a heater driving heat enable terminal, a data input terminal, a latch, a clock, a logic power source, a temperature sensor terminal, a rank measurement terminal, and so on.
In recent years, there is a demand for significant improvements in recording resolution and recording speed of an ink jet recording apparatus. This creates the need for an ink jet recording head substrate that is capable of high-density placement of heaters and logics and also has a greater length to increase the number of heaters itself. At the same time, enhanced functionality of the substrate itself is also desired, so that the inclusion of a fuse circuit, the provision of a plurality of temperature sensors for finer temperature control, and the like are necessary.
To realize the above-mentioned structure, the number of connection pads to the outside needs to be increased. In detail, as the number of necessary heaters increases, the logic circuit increases, which leads to an increase in the number of connection pads to outside the substrate. In addition, to achieve the above-mentioned functionality enhancement, pads for fuse reading/writing and output pads of temperature sensors need to be added.
This causes a problem that pads cannot be contained in a connection pad area at the substrate end. The pad area insufficiency tends to be significant particularly in a substrate for black ink which has one row of ink supply port and also has a large number of nozzles, due to a small width of the substrate.
When the substrate size on the shorter side where pads are arranged is increased as shown in FIG. 10, necessary pads can be contained. However, this causes a larger substrate size, and an increase in cost.
On the other hand, when necessary connection pads 701 are arranged on a longer side of the substrate as shown in FIG. 11, spaces 702 on the longer side other than the pads are wasted, causing a larger substrate size. This is because a logic circuit and wiring cannot be positioned under connection pads, and so the conventional substrate needs to be widened to use portions under which nothing is provided, as pads. Besides, the presence of connection pads on the longer side of the substrate is not preferable as it interferes with a wiping operation of wiping off ink adhering to a nozzle surface.
A method of reducing the pad size or arrangement pitch to enable more pads to be provided may also be contemplated. This method, however, tends to cause decreases in yield and implementation reliability as there is a high possibility of a short circuit occurring in wiring connections, and therefore has only a limited effect. Moreover, when the pad arrangement pitch is reduced, the width of power source wiring connected to the pad is reduced too, which induces a problem of a wiring resistance increase.